My subject had no trouble assuming a serious pose--other than that he kept wanting to get too close to my seamless paper background:
But, whenever I tried to get him to loosen up, he remained very stiff in front of my camera. This next shot is the best "smile" that I could coax out of him.
Perhaps he's self conscious about his teeth? Or, I suspect it's more my fault than his. It's the photographer's job to make the subject comfortable during a photo shoot.